Answer: Distances are measured from the optical centre; those measured in the direction of the incident light are positive and those against it are negative.
- The object is always placed on the left of the lens, so the incident light travels from left to right. All distances are measured from the optical centre of the lens.
- Distances measured in the direction of the incident light (to the right) are taken as positive, and those measured against it (to the left) as negative. So the object distance u is always negative.
- Distances measured upwards from the principal axis are positive and those measured downwards are negative (heights of the object and the image). The focal length of a convex lens is positive and that of a concave lens is negative.